Digital Power Supply

ABSTRACT

Provided is an apparatus and method for a digital power supply that can provide independent power control, and control variable power, for two or more electrical loads. Disclosed embodiments may reduce the magnitude of harmonic currents and/or flicker introduced into a power system. Embodiments include a microprocessor that delivers power to electric loads using phase-controlled AC current. The microprocessor may calculate a power array corresponding to a requested power for each electric load. Logic is provided for populating the power array in a pattern that reduces the magnitude of harmonic currents and flicker.

FIELD OF THE INVENTION

The present inventions relate to a digital power supply forindependently controlling two or more high-powered loads with reducedharmonic and flicker introduction. In a non-limiting embodiment, adigital power supply may be used in an electric grill to independentlycontrol two or more heating elements while reducing harmonics andflicker introduced to the power system.

BACKGROUND OF THE INVENTIONS

There is an increasing desire for a power supply that can independentlycontrol two or more high-powered loads using an AC wall outlet whileintroducing a reduced amount of harmonics and/or flicker into the powergrid. The urban population is increasing, and with it there is anincreasing desire for high powered loads that can be plugged into an ACwall outlet. By way of example, urban dwellers live in apartment orcondominium buildings where they would like to use a grill. Because ofsmoke, gas, or other concerns, use of typical charcoal or gas grills maynot be permitted or desirable.

There are a number of available electric cooking devices, such as theGeorge Foreman Plate Grill (and similar devices), Panini presses,electric griddles and the like. However, these prior art devicesgenerally do not deliver variable power. Moreover, these prior artelectric cooking devices typically cannot generate enough power to matcha gas or electric grill.

Some prior art devices may use variable resistors in series withelectric loads to control an amount of power delivered to the load. Forexample, as the resistance of a variable resistor increases, thevariable resistor restricts power from being delivered to an electricload. The use of variable resistors to control power delivery toelectric loads is well known. But variable resistors come withdisadvantages. For example, disadvantages may include the introductionof harmonics onto the electrical system, which translates toelectromagnetic emissions that can create interference and otherunpredictable electromagnetic fields. Moreover, variable resistors maybe inefficient because they burn a lot of power.

Other prior art devices may use a bi-metal thermometer which opens andcloses to control power delivery. Disadvantages of using a bi-metalthermometer include the fact that it allows for less discrete (i.e.,less precise) control over power delivered and is usually associatedwith a relatively long lag in response time. A long lag time causes anegative cooking experience because it leads to poor control overtemperature. Moreover, a long lag time is disadvantageous because longon/off duty cycles are known to shorten the life span of a heatingelement

Some devices may use half-wave control techniques to deliver power. Forexample, U.S. Pat. No. 6,772,475, titled “Heating Control System WhichMinimizes AC Power Line Voltage Fluctuations,” discloses half wave ACcontrol devices to control delivery of AC current. This control methodis associated with significant disadvantages because it delivers poweronly in stages, not in a continuous range from 0-100%. By contrast,embodiments of the present invention allow continuous variable powerdelivery.

Yet other prior art devices may include a digital control for limitingthe in-rush of electric current when an electric load in turned on. Forexample, U.S. Pat. No. 6,111,230, titled “Method and apparatus forsupplying AC power while meeting the European flicker and harmonicrequirements,” describes a method for limiting the in-rush of current toa printing device when it is first turned on. However, the discloseddevices do not provide for independently controlling multiple electricloads, much less for reducing harmonic currents and flicker whileindependently controlling multiple loads.

Thus, there is a need for a digital power supply that can independentlycontrol two or more electric loads while introducing only reducedharmonic and flicker interference to the power system.

BRIEF SUMMARY OF THE INVENTIONS

The present inventions overcome many of the deficiencies of known powersupplies and provide new features and advantages for devices such aselectric grills. For example, embodiments of the present inventionprovide digital power controls that can deliver more precise amounts ofpower to electric loads. Moreover, embodiments of the present inventionallow a plurality of electric loads to be controlled independently. Yetfurther embodiments of the present invention reduce the harmoniccurrents and flicker that may result from plugging a power supply into awall outlet.

In accordance with a preferred embodiment of the present invention, amethod of delivering power is provided. The method may include the stepsof using one or more user input devices to select a first and secondpower setting for a first and second heating element, respectively;electronically communicating the power settings to a microprocessor andusing the microprocessor to calculate a total amount of power requested;using the microprocessor to populate a first and second power arraycorresponding to the first and second heating element, respectively;using the microprocessor to calculate a first and second phase anglearray corresponding to the first and second power array; causing themicroprocessor to receive a zero crossing signal from a zero crossingdetection unit; and for a first time period, delivering aphase-controlled AC wave pattern represented by the first phase anglearray to the first heating element and delivering a phase-controlled ACwave pattern represented by the second phase angle array to the secondheating clement. Additional embodiments of the inventions comprise thestep of, for a second time period, delivering a phase-controlled AC wavepattern represented by the first phase angle array to the second heatingelement and delivering a phase controlled AC wave pattern represented bythe second phase angle array to the first heating element.

Further, each power array may contain four cells. Additionally, the stepof using the microprocessor to populate a first and second power arraymay further comprise the steps of populating the first cell of the firstpower array with the same value as the third cell of the first powerarray; populating the second cell of the first power array with the samevalue as the fourth cell of the first power array; populating the firstcell of the second power array with the same value as the third cell ofthe second power array; and populating the second cell of the secondpower array with the same value as the fourth cell of the second powerarray.

In embodiments of the inventions, each cell of each power arrayrepresents a power percentage and ranges from 0≦x≦1.0. Every alternatecell in the first power array may be populated with a “0” or a “1”.Moreover, every alternate cell in the second power array may bepopulated with a “0” or a “1”. In some embodiments of the inventions,the first and second phase angle arrays are calculated using themicroprocessor to apply the equation angle=arccos(2x−1) to the first andsecond power array, respectively. In further embodiments, the first timeperiod is calculated as a ratio of the first power setting to the totalamount of power requested and the second time period is calculated as aratio of the second power setting to the total amount of powerrequested. Further, embodiments may include the step of activating atriac connected to a heating element.

Also provided are embodiments of a digital power supply, having a firstand second user input; a first and second triac connected to a voltageline; a first and second triac driver respectively in communication withthe first and second triac; a microprocessor in communication with thefirst and second triac drivers and in communication with the first andsecond user input; wherein the microprocessor is specifically configuredto calculate a total power requested by the first and second user inputsand to populate a first and second power array based on the total powerrequested; and wherein the microprocessor is specifically configured tocalculate a first and second array of phase angles based on therespective values of the first and second power array.

In embodiments of the invention the first and second power array eachhave four cells. The microprocessor may be specifically configured topopulate at least one power array's cells with two alternating values.In further embodiments, the microprocessor may be configured turn on thefirst and second triacs in a timing pattern that corresponds to aphase-controlled wave form in the first and second phase angle arrays.

Still further embodiments include an electric grill, having a firstknob, a second knob, and a display mounted on a housing; a power cableconnected to a voltage line and a neutral line; a first and secondheating element inside the housing, the first and second heatingelements being connected to the voltage line and the neutral line; afirst and second triac connected between the voltage line and the firstand second heating elements respectively; a first and second triacdriver respectively in communication with the first and second heatingelements; a zero crossing detection unit configured to detect zerocrossings of AC current in the voltage line; and a microprocessor incommunication with the first and second knob, the first and second triacdrivers, and the zero crossing detection unit, wherein themicroprocessor further communicates with a clock signal generator and amemory.

Moreover, in some embodiments the memory contains a first and secondpower array. The first power array may be populated with two alternatingvalues. The second power array may be populated with two alternatingvalues. One of the two alternating values in the first power array mayrepresent a full “on” wave. In yet further embodiments, one of the twoalternating values in the first power array may represent a full “off”wave.

Accordingly, it is an object of the present invention to provide adigital power supply that provides precise power control, mayindependently control multiple loads, and may reduce harmonic currentsand flicker introduced by the power supply into a wall outlet.

Another object of the invention is to provide an improved power supply,including but not limited to one that may be used with an electricgrill.

It is an additional object of the invention to provide a digital powersupply that can be used in an electric grill to provide independentcontrol over two or more heating elements.

It is an additional object of the invention to provide a digital powersupply that introduces fewer harmonic currents into a wall outlet.

It is an additional object of the invention to provide a digital powersupply that introduces less flicker into a wall outlet.

It is an additional object of the invention to provide a digital powersupply for use in an electric grill that complies with standard limitsand/or regulations on harmonic currents and flicker.

It is an additional object of the invention to provide a digital powersupply for use in an electric grill to deliver variable power to two ormore heating elements.

It is an additional object of the invention to provide a digital powersupply that uses phase cutting techniques to deliver variable power.

It is an additional object of the invention to provide a digital powersupply that delivers continuous variable power in a range of 0-100%.

It is an additional object of the invention to improve a heatingelement's life span by providing short duty cycles.

Inventors' Definition of Terms

The following terms which may be used in the various claims orspecifications of this patent are intended to have their broadestmeaning consistent with the requirement of law:

As used herein, a “power array” is defined to be an array of values,each value representing a percentage (0.0≦x≦1.0) of power delivery inone wave cycle. Exemplary power arrays are described as having fourcells, but it should be understood that arrays of other sizes arepossible.

As used herein, a “phase angle array” is defined to be an array ofvalues, each value representing the phase angle “cut” in one wave cycle.Exemplary phase angle arrays have four cells, but it should beunderstood that arrays of other sizes are possible.

As used herein, a “timing pattern” is defined to be a pattern of “on”and “off” signals that create phase-controlled AC wave forms.

Where alternative meanings are possible, in either the specifications ofclaims, the broadest meaning is intended consistent with theunderstanding of a person of ordinary skill in the art. All of the wordsused in the claims are intended to the use in the normal, customaryusage of grammar, the trade and the English language.

DRAWINGS

FIG. 1A is a front view of an exemplary grill of the present inversion.

FIG. 1B is a top schematic view of a cooking surface of a representativegrill showing representative internal components.

FIG. 2 is a schematic of an exemplary embodiment of a circuit, includinga digital power supply circuit of the present invention.

FIG. 3A is an exemplary wave form with a 90 degree cut of the presentinvention.

FIG. 3B is an exemplary wave form with a 90 degree cut of the presentinvention.

FIG. 3C shows harmonic currents plotted against standard limits showingharmonic currents by a 1150 W element.

FIG. 4A is an exemplary cut wave form followed by an “on” wave of thepresent invention.

FIG. 4B is an exemplary cut wave form followed by an “on” wave of thepresent invention.

FIG. 4C shows harmonic currents plotted against standard limits showingharmonic currents by a 1150 W element.

FIG. 5A is an exemplary cut wave form followed by an “off” wave of thepresent invention.

FIG. 5B is an exemplary cut wave form followed by an “off” wave of thepresent invention.

FIG. 5C shows harmonic currents plotted against standard limits showingharmonic currents by a 1150 W element.

FIG. 6 is a flow chart of an exemplary microprocessor configuration ofthe present invention.

FIG. 7 is an exemplary algorithm for populating a power array of thepresent invention.

FIG. 8 shows exemplary power delivered to two heating units over aperiod of time of the present invention.

FIG. 9 shows exemplary power delivered to n-number of heating units ofthe present invention.

FIG. 10 is a flow chart of exemplary inputs and outputs to amicroprocessor of the present invention.

FIG. 11 shows standard (IEC 61000-3-3) limits for flicker.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Set forth below is a description of what is currently believed to be thepreferred embodiments or best representative examples of the inventionsclaimed. Future and present representative or modifications to theembodiments and preferred embodiments are contemplated. Any alterationsor modifications which make insubstantial changes in function, purpose,structure or result are intended to be covered by the claims of thispatent. The present inventions may be used on and/or part of electricgrills with current protection circuitry as discussed in the co-pendingpatent application entitled “Electric Grill With Current ProtectionCircuitry” filed by Applicants on the same day as this application andassigned to Weber-Stephen Products LLC, and which is incorporated hereinby reference in its entirety.

The present inventions generally include a digital power supply that canprovide independent power control, and continuous variable power, fortwo or more electrical loads. Embodiments of the present inventions mayreduce the amount of harmonics and/or flicker introduced into a powersystem. A person of ordinary skill in the art would recognize that thedigital power supply may be used to supply any electrical load orcombinations of loads, including heaters, motors, and the like. In apreferred embodiment described herein, exemplary loads are heatingelements found in an electric grill.

Electric grills are a suitable application for a digital power supplywith independent load control because a user may wish to have higherheat on one side of an electric grill and lower heat on the other sideof the grill. Such an arrangement allows a user to simultaneously grillvarious foods requiring different temperatures, or to use indirectgrilling methods. Examples of indirect grilling methods include placingfoods on one side of a cooking surface while heating another side,thereby avoiding direct contact between the food and the heat source. Afurther benefit of variable power is that it allows a user to input apower setting and achieve targeted temperatures. This makes it possibleto cook at low temperatures for prolonged periods of time.

Referring now to the drawings, FIGS. 1-11 show preferred embodiments ofan electric grill 110 and a digital power supply 200. By way of example,FIGS. 1A and 1B show an electric grill 110. FIG. 1A shows the exteriorof electric grill 110, including a housing 106, onto which left andright control knobs 101 and 102, as well as display 103, may be mounted.The electric grill 110 may include a power cord 107 for connecting to anAC wall outlet. Left and right control knobs 101 and 102, and display103, may connect to a microcontroller 213 which is described in greaterdetail herein.

As shown in FIG. 1B, left and right control knobs 101 and 102 may beassociated with a first and second heating element, 203 and 204,respectively, thus creating dual cooking zones. A representative grateor cooking surface 112 is also shown in FIG. 1B. Each heating element203 and 204 may be controlled independently by a knob 101, 102 or anyother controller associated with the heating element 203, 204. Left knob101 and tight knob 102 may be positioned on the exterior of a grillhousing 106. The knobs 101 and 102, or any other input device that willbe understood by those of skill in the art, may be connected to amicroprocessor 213 to set the operating mode of one or more heatingelements 203, 204.

Using knobs 101 and 102, or any other input device such as a touchscreen or buttons, a user may select an operating mode for each heatingelement 203 and 204. The operating mode may include a desiredtemperature or power setting for the heating element. Microprocessor213, described in further detail herein, controls the electric currentdelivered to heating elements 203 and 204 in order to deliver theselected power. Microprocessor 213 can achieve a desired temperature foreach heating element 203 and 204 using a feedback loop in which itreceives a current temperature reading from thermocouples 221 and 222,which are proximally positioned by respective heating elements 203 and204. A person of ordinary skill in the art would recognize that varioustypes and numbers of knobs, heating elements, temperature sensors and/ordisplays may be used.

The electric grill 110 may optionally include a display 103 or otheruser interface. In one example the display 103 may be connected tomicroprocessor 213 and display information relating to the currentsettings or operation of one or more of the heating elements 203, 204.For example, the display 103 may show the current temperature in theproximity of heating elements 203 and 204 (as measured by thermocouples221 and 222), as well as the desired temperature or power setting a userhas selected via knobs 101 and/or 102.

Turning now to FIG. 2, in general, non-limiting terms, digital powerdelivery may be accomplished by a microprocessor 213 which receives auser's desired power setting(s) and controls triacs 208 and 209 toenable (or disable) AC current to flow from voltage line 201 throughheating elements 203 and 204 and return to a wall outlet through neutral202. Additionally provided herein is a specifically configuredmicroprocessor 213 which may control the flow of AC current to theheating elements 203 and 204 in a manner that reduces the amount ofharmonic current and flicker introduced by the electric grill 110 to theAC wall outlet.

As shown in the embodiment of FIG. 2, microprocessor 213 is incommunication with triac drivers 211 and 212, which in turn controlrespective triacs 208 and 209. The mechanism by which microprocessor 213may deliver power to heating elements 203 and 204 is by turning triacs208 and 209 on or off (sometimes referred to as “enabled” and“disabled,” respectively) via their corresponding triac drivers 211 and212.

Specifically, triacs 208 and 209 turn “on” when they are triggered by apulse from microprocessor 213. Current continues to flow until an ACcurrent wave crosses zero. After a zero crossing, a triac turns off andremains off until the next time microprocessor 213 turns it on. In anexample where AC current is 60 Hz, such as a typical wall outlet, a zerocrossing occurs every 1/120^(th) of a second. A zero crossing detectionunit 210 is provided to communicate a signal to microprocessor 213 eachtime an AC wave crosses zero. Using this signal, microprocessor 213 cansynchronize its timing to the alternating current's zero crossings.

Instead of permitting direct communication between microprocessor 213and triacs 208 and 209, triac drivers 211 and 212 are used to interfacebetween microprocessor 213 and triacs 208 and 209. Triac drivers cancontrol a high voltage triac with a low voltage DC source (such as amicroprocessor) (FIG. 2). Moreover, triac drivers are used to isolatedevices from a potentially high current or voltage in a triac. Triacdrivers 211 and 212 interface between microprocessor 213 and triacs 208and 209 while at the same time keeping microprocessor 213 isolated fromvoltages and currents in triacs 208 and 209.

An “on” triac allows current to flow through it, whereas an “off” triacdoes not allow current to flow. Thus, an “on” triac 208 permits ACcurrent to flow (from voltage line 201) through first heating element203 and an “on” triac 209 permits AC current to flow (from voltage line201) through second heating element 204. To say that microprocessor 213delivers power to a heating element 203 and/or 204 implies thatmicroprocessor 213 enables the respective triac driver, which turns therelevant triac “on” and allows AC current to flow from line 201.Throughout this disclosure, it should be understood that references tomicroprocessor 213 delivering power to a heating element mean thatmicroprocessor 213 is activating a given heating element's triac Drivervia an “on” or “enable” pulse signal.

As a person of ordinary skill would understand, triacs are threeelectrode devices, or triodes, that conduct alternating current. Triacsare a type of solid state bidirectional switch. Although this disclosuredescribes a digital power supply that uses triacs, it should beunderstood that any solid state bidirectional switch may be used insteadof a triac. Heating elements 203 and 204 may be resistive heaters whichincrease in temperature as more current passes through them. Exemplaryheating elements may draw 1150 Watts. Other heating elements 203, 204may also be used as will be understood by those of skill in the art.

In embodiments of the invention, microprocessor 213 may optionallyreceive temperature feedback from one or more thermocouples 221 and 222located proximately to each heating element 203 and 204 in order torecognize when a desired temperature has been achieved. FIG. 1B shows anexample of thermocouples 221 and 222 adjacent to each heating element203 and 204. In an embodiment, the feedback may be used bymicroprocessor 213 to adjust the current delivered to the heatingelements 203 and 204 until the desired temperatures selected by knobs101 and/or 102 is achieved. As a result, a user can (independently)select a desired operating mode for heating elements 203 and 204. Inembodiments of the invention, microprocessor 213 may control the currentdelivered until a desired temperature setting is reached and thenmaintain the desired temperature.

Turning next to the operation of microprocessor 213, microprocessor 213may be configured to deliver an appropriate amount of power (as selectedby the user) by toggling triacs 208 and 209 between “on” and “off.” Asdescribed above, an enabled (or “on”) triac 208 or 209 allows AC currentto flow from line 201 through heating elements 203 or 204, respectively.Therefore it follows that a longer “on” period allows more AC current toflow and therefore delivers more power. Conversely, a longer “off”period results in lower power delivery.

In embodiments of the invention, microprocessor 213 may use phase anglecontrol techniques to create a pattern of toggling between “on” and“off.” The control pattern created by toggling between “on” and “off”controls the phase angle of AC current (and by extension, power) flowingfrom voltage line 201 through heating elements 203 and 204. This type ofcontrol pattern is sometimes referred to as “phase cutting,” because ACcurrent's wave forms may be “cut” off. Waves are cut by disabling theflow of current during part of an AC wave cycle. In this way, part ofthe wave becomes “cut” off. The timing pattern of “on” and “off” createsa phase-controlled wave. To determine the correct angle at which to cuta wave for a desired power delivery, microprocessor 213 solves theequation:

(angle)−arccos(2x−1)

where x is the desired powder delivery (expressed as a percentage:0.0≦x≦1.0). Microprocessor 213 is programmed to solve for the angle atwhich to cut the AC sine wave delivered to heating elements 203 and 204.This disclosure refers to angles in “degrees,” but a person of skill inthe art would understand that every angle measurement may be convertedinto the unit “radians.”

An example is provided in FIG. 3A, which illustrates an example wheremicroprocessor 213 cuts an AC wave at 90°. A 90° cut produces a wavethat delivers half (i.e. 50%) of the total available power. FIG. 3Ashows one wave cycle of an AC current. A person of skill in the artwould understand that a complete wave has a positive half and a negativehalf. The wave cycle begins at 301 where the current's value is zero.The area between 301 and 303, numbered 302 is shaded gray to indicate atriac is not enabled and therefore current is not being delivered. At303, which represents a 90° phase angle, microprocessor 213 sends apulse signal to activate a triac and thus allow current to flow througha heating element. (Stated differently, microprocessor 213 beginsdelivering power at 303). At 305, current crosses zero and the triacturns off. The triac remains off until 307, which represents a 270°phase angle. At 270°, microprocessor 213 again sends an activating pulseand current flows for a 90° phase, between 307 and 309, i.e. from 270°to 360°.

In sum, FIG. 3A shows microprocessor 213 delivering power for the areasmarked 304 and 308, each representing 90° phases, for a combined 180°.No power is delivered for the shaded areas marked 302 and 306, alsorepresenting 90° each, for a combined 180°. In this way, microprocessor213 has delivered half, or 50%, of the power that was available. Todeliver a different power percentage, microprocessor 213 may send anactivating pulse earlier in a half-wave to deliver more power, or laterin a half wave to deliver less power. For any desired power percentage,the appropriate phase angle cut may be calculated by microprocessor 213solving for (angle)=arccos (2x−1). In the example of FIG. 3A, a 50%power deliver was selected. Therefore, microprocessor 213 executed thecalculation (phase angle)=arccos (2*0.5−1)=90°. FIG. 3B removes the “cutoff” wave portions of FIG. 3A and shows only the power actuallydelivered.

Turning now to FIG. 3C, a graph is provided which shows the harmoniccurrents introduced into a power system by a 90° phase cut described inFIGS. 3A and 3B. In other words, these plotted harmonic currents may beintroduced into a building's power lines when an electric grill isplugged into a wall outlet and makes the 90° phase cut described inFIGS. 3A/3B. The plot is made using a 1150 W heating element.Introducing harmonics is undesirable because it leads to electromagneticinterference. Moreover, there are standards, such as IEC 61000-3-2Electromagnetic compatibility (EMC)-Part 3-2, which limit the level ofharmonic currents that may be introduced into a wall outlet by a device.The harmonic current limits are plotted as line segments in the graph ofFIG. 3C. As will be clear from FIG. 3C, the harmonic currents (plottedas points) introduced by the 90° phase cut exceed the harmonic limits(plotted as the segments). In other words, the graph in FIG. 3C showsthat the points (representing the RMS current) are higher than the lineswhich mark the harmonic limits. This means that the wave forms of FIGS.3A/3B have high harmonic currents and do not comply with the IECstandard. For example, RMS current at point 310 is one example of aharmonic current that exceeds (i.e., is above) the harmonic limit 311.

Therefore, embodiments of the inventions include a microprocessor 213specially configured to deliver power to electric loads using wave cutsthat induce harmonic currents having reduced magnitudes. As an initialmatter, Applicants' testing has shown harmonic currents' magnitudes arereduced when a wave cut is immediately followed by a full wave cycle“on” or a full wave cycle “off,” Applicants' test results are shown inFIGS. 4 and 5. In particular, FIG. 4A shows a first wave cycle havingthe same 90° cut as in FIG. 3A, but is followed by a subsequent secondwave cycle (between 409 and 410) that is fully “on.” Similarly, FIG. 5Ashows a first wave cycle having the same 90° cut as FIG. 3A, andadditionally followed by a second full wave cycle (between 509 and 510)that is fully “off.” For clarity, FIGS. 4B and 5B show the samerespective patterns without the “cut” portions of a wave. Applicants'testing, shown in FIGS. 4C and 5C, shows that a 90° cut induces fewerharmonics when it is followed by a subsequent full “on” or a full “off”wave cycle. These results can be seen in FIGS. 4C and 5C, where theplotted harmonic currents (points) are now below the harmonic currentlimits of the IEC standard (plotted as line segments), and arenoticeably lower than the harmonic currents plotted in FIG. 3C. By wayof example, FIG. 4C shows an exemplary RMS current point 410 that isbelow the harmonic limit 411. Unlike FIG. 3C, the RMS currents of FIG.4C are under the harmonic limits. The same applies to FIG. 5C, whereexemplary current point 510 is under the harmonic limit of 511.

Therefore, embodiments of the inventions include a microprocessor 213specifically configured to follow a cut wave with either a full “on” ora full “off” wave. Moreover, microprocessor 213 may be specificallyconfigured to draw current in a pattern that reduces harmonic currentswhile still managing to split the drawn current among two independentheating elements 203, 204. In other words, microprocessor 213 mustmanage the pattern of the overall current drawn by the electric grill110 while simultaneously satisfying the power requirements of bothindependent heating elements 203, 204. The pattern of the overallcurrent drawn by electric grill 110 may be referred to as the electricgrill 110's total power array. The electric grill 110's total powerarray is the sum of the first heating element 203's power array plus thesecond heating element 204's power array. An exemplary power array maybe four cells, each cell containing a value (0.0≦x≦1.0) representing apercentage of power to deliver in a wave form. Thus, an exemplary powerarray may represent a pattern of four waves. It will be understood thatthe total power (or, current) drawn by electric grill 110 is the sum ofthe power (current) drawn by the heating elements. The wave formpatterns delivered to the heating elements 203, 204 may likewise berepresented as four-celled power arrays. The first heating element'spower array summed with the second heating element's power array equalsthe electric grills total power array. The same holds true for anynumber of heating elements in an electric grill 110. The electric grill110's harmonic currents depend on the pattern of waves drawn by theelectric grill 110, represented in the total power array. To reduceharmonic currents, electric grill 110's total power array shouldrepresent a pattern where each “cut” wave is followed by a full “on” ora full “off” cycle.

FIG. 6 is a flow chart showing an exemplary configuration ofmicroprocessor 213 for controlling two heating elements whileintroducing fewer harmonics. Generally speaking, microprocessor 213calculates a power array to deliver to each heating element 203, 204.The power arrays depend on a user's power settings for each of the twoheating elements 203, 204 as well as feedback from thermocouples 221 and222. In this example, each power array consists of four cells (butanother number of cells may be used), each cell containing a numberranging between 0.0≦x≦1.0. Each of the four cells represents a wavecycle, the cell's number indicating the percentage of power deliveredduring that wave cycle. By way of example, an array of “1|0|1|0” wouldrepresent one “on” wave, one “off” wave, another “on” wave, and another“off” wave. Microprocessor 213 delivers the wave forms from the twocalculated power arrays to the two heating elements 203, 204 by togglingthe triac drivers 211 and 212 in the manner described above.

Addressing FIG. 6 more particularly, microprocessor 213 communicateswith a first and second user input device, such as a left knob 101 and aright knob 102. The first and second user input devices convey a powerlevel for each of the two heating elements 203, 204. The desired powerlevels can be converted by microprocessor 213 into a percentage of totalpower at steps 601 and 602. Microprocessor 213 determines if the totalpower 603 is greater than or equal to 50% at step 604.

At 605, where a user's selected total power is less than 50%,microprocessor 213 begins filling (or, “populating”) the cells of thefirst power array. FIG. 7 shows the steps microprocessor 213 isconfigured to execute to fill, or populate, a power array. As seen inFIG. 7, microprocessor 213's calculation begins at 701 with the totalpower requested by a user. (This is the sum of the power requested forthe right heating element and the power requested by the left heatingelement as determined in 603). The percentage of total power requestedis multiplied by 8 (because there are 2 arrays×4 cells each) at step702. The value of step 702, herein referred to using the notation [702],is used to populate a power array at 703. If the value of 702 is lessthan or equal to 2.0, the value of 702 is distributed evenly between thefirst and third array elements to arrive at: “([702]/2)|0|([702]/2)|0.”This is seen at step 704. If the value of 702 is greater than 2.0, thenthe first and third array elements are filled with “1,” and theremainder (subtracting 2 from the value of 702) is distributed evenlybetween the second and fourth cells. This is seen at 705. Using thistechnique, a power array is constructed to have a full “on” or a full“off” wave that follows a cut wave to reduce the magnitude of harmoniccurrents. Moreover, the power array's alternating pattern reducesflicker, as described in more detail below. Returning now to FIG. 6, thesecond power array is filled with four zeros: “0|0|0|0” at step 606.

Again by reference to FIG. 6, if the Total Power 603 equals or exceeds50%, microprocessor 213 fills the first power array with all 1's(“1|1|1|1.”) at step (607). Microprocessor 213 then populates the secondpower array using the conditions of 706 and 707. Regardless of whetherthe user has requested more or less than 50% power, one of the two powerarrays will have the alternating pattern “A|B|A|B,” while the otherarray will have the pattern “C|C|C|C,” where C=0 or 1. Once the firstand second power array have been populated, they are delivered to theheating elements 203 and 204.

Power is delivered by microprocessor 213 to a triac driver based on thevalues in the four cell power arrays. As described above, each cellrepresents one full wave cycle, and the cell's numeric value representsthe percentage of power to deliver in that wave cycle. As also describedabove, embodiments of the inventions may use phase cutting techniques tocontrol power. Thus, at step 609, microprocessor 213 is configured tocalculate the phase angle at which to “cut” a wave in order to achievethe power represented by a cell in a power array. Microprocessor 213 isconfigured to solve the equation:

(angle)=arccos(2*power−1),

where “power” is the power represented by a number in a power array'scell. Microprocessor 213 uses this angle to deliver a wave cycle havingpower that corresponds to the cell's numeric value. The calculation maybe repeated for each cell in each power array. Each cell of each powerarray may be converted into a corresponding phase angle 610 and 611. Thecorresponding phase angle arrays contain phase angles, rather than powerpercentages, and may be stored in the same format at the power arrays.

At step 614, microprocessor 213 may synchronize its timing to the phaseangle of AC current in line 201. As described above, microprocessor 213receives a zero crossing signal from zero crossing detection 210 eachtime the AC current crosses zero from zero crossing detection unit 210.The zero crossing signal can thus synchronize microprocessor 213'stiming (and therefore by extension, the angle) of an AC wave. Forexample, a person of skill in the art would then recognize that a waveof AC current has the following angles at the indicated points in time:

TABLE 1 Desired phase 60 Hz AC current: Time angle “cut” (where zerocrossing is t = 0)  0°       0 seconds 10° 0.000462963 seconds 20°0.000925926 seconds 30° 0.001388889 seconds 40° 0.001851852 seconds 50°0.002314815 seconds 60° 0.002777778 seconds 70° 0.003240741 seconds 80°0.003703704 seconds 90° 0.004166667 seconds

Using this information, microprocessor 213 may use an internal timingmechanism, such as a clock signal generator or any other appropriatemechanism, to send the “on” or “enable” pulse at an instancecorresponding to the angle required for the correct “cut.” For example,Table 1 shows that a 90 degree cut would be made by activating a triac0.00416666 seconds after a zero crossing. Microprocessor 213 may use aclock signal to enable a triac at the appropriate point in time. Aperson of skill in the art reading this disclosure would understand howto calculate the timing for any desired wave “cut.”

Turning now to steps 612 and 613, the first power array is delivered tothe first triac driver 211 and the second power array is delivered tothe second triac driver 212 for a period of time equal to T1. This powerdelivery continues repeatedly for a first time period T1, after whichmicroprocessor 213 delivers the first power array to the second triacdriver 211 and delivers the second power array to the first triac driver212 repeatedly for a second time period T2. After T1, delivery is“flipped” and the first triac driver 211 receives the second power arrayfor duration of T2. The first and second power array, summed together,equal the electric grill 110's total power array—thus, by definition,the first and second power array must always be deliveredsimultaneously.

The discussion now turns to the calculation of time periods T1 and T2 at615 and 616. The purpose of time periods T1 and T2 is to “split,” orpro-rate, the total power drawn by the electric grill (or any otherdevice using embodiments of the invention) between the two heatingelements (or any other electric load) according to the independentlyselected power for each respective heating element. The power arrayscreated at steps 605 through 608 create an acceptable wave pattern forthe electric grill as a whole. The sum of the power arrays, which is theelectric grill 110's total power array, will have a full “on” or full“off” wave following each cut wave, which reduces the magnitude ofharmonic currents. It is additionally necessary to calculate thedelivery time of each power array to the respective heating elements203, 204.

The time period T1 is calculated by taking the power setting for thefirst heating element 203 and dividing it by the total power selected,603. That ratio is then multiplied by the power delivery phase, which is2 seconds in this example but may be varied, T1 and T2 are simple ratiosof a given heating element's power setting compared to the totalrequested power. The calculation may be summarized by the followingequation:

T1=2 seconds*(power selection for first heating element)/((powerselection for first heating element)+(power selection for second heatingelement)).

Similarly, T2 is the same calculation, this time for the second heatingelement 204;

T2=2 seconds*(power selection for second heating element)/((powerselection for first heating element)+(power selection for second heatingelement)).

FIG. 8 summarizes microprocessor 213's power delivery of the first andsecond power array to the first and second triac drivers over a powerdelivery phase of 2 seconds: the first triac driver 211 (and byextension first heating element 203) receives the waves represented bythe first power array for a time T1. It then receives waves representedby the cells of the second power array for a time T2. Conversely, thesecond triac Driver 212 (and by extension the second heating element204) receives waves represented by the cells of the second power arrayduring the time period T1, and then receives waves represented by thecells of the first power array during the time period T2.

Embodiments of the present invention may be scaled to independentlydeliver power to more than two loads. In an embodiment where a digitalpower supply independently controls “n” number of loads, n power arraysare required. Moreover, the decision at 604 would compare the totalpower to 100%/n. The technique for filling the power arrays of FIG. 7remains applicable, although rather than multiply by eight (8), it wouldbe necessary to multiply step 702 by (n*4). Moreover, at steps 615 and616, n time periods are required. FIG. 9 shows the timing of n-powerarrays delivered across n-time periods. It should be understood thatembodiments with multiple heaters without independent control are alsocontemplated by this disclosure.

The present inventions also provide methods for independentlycontrolling two heating elements and providing variable power whileproviding reduced harmonic currents and flicker. In an embodiment of theinvention, a user activates electric grill 110 and selects a first andsecond power level, for example by controlling knobs 101 and 102. Byactivating an electric grill 110, a user controls microprocessor 213 toexecute the following steps for the benefit of controlling one or moreheating elements. It is understood that some embodiments may include anynumber of knobs or other user inputs. By activating the electric grill110, a user turns on microprocessor 213. Microprocessor 213 receives theuser's selected power settings and performs the above-describedcalculations to activate triac drivers 211 and 212 in a control patternthat delivers phase-controlled wave forms to heating elements 203 and204.

In embodiments of the invention, microprocessor 213 performs the step ofcalculating the appropriate phase controlled wave forms by populatingtwo power arrays 605-608. Each power array may have four cells. Eachcell contains a number “n,” where 0.0≦n≦1.0. The number “n” represents awave form having “n”-percentage of power. The waves are cut to eliminate“excess” power. Microprocessor 213 performs the step of filling in thepower arrays by calculating the total power requested by all heatingelements 203, 204, which may be expressed as a percentage of selectedpower as compared overall available power (in decimal form).

If the total power requested (i.e. the total requested power for allheating elements) by the user is less than 50% of the overall availablepower, microprocessor 213 performs the step of filling in the firstpower array (605). The power array is populated by distributing thetotal power number into the power arrays four cells. At 606,microprocessor 213 performs the step of filling all zeros into thesecond power array (i.e. “0000”). If the total power requested by theuser is greater than, or equal to, 50% of the overall power,microprocessor 213 performs the steps of fillings the first power arraywith 1's (i.e. “1|1|1|1”) and the second power array is filled (withTotal Power—50%, i.e. [702] minus 4) according to the steps of FIG. 7.

Once the first and second power array are calculated, microprocessor 213delivers wave forms corresponding to the cells of each power array. Inparticular, each cell's value represents the percentage of power todeliver in one wave cycle. To deliver a wave having any given percentageof power, microprocessor 213 calculates a phase angle=arccos(2*x−1),where x is the power percentage described in any given cell.Microprocessor 213 uses the calculated angle to deliver an “on” signalto triac Drivers 211 or 212 at a point in time corresponding to thecalculated phase angle. Microprocessor 213 may use a zero crossingsignal and the above-described Table 1 to determine the correct timing.

Microprocessor 213 repeatedly delivers the first power array to thefirst triac driver 211 and the second power array to the second triacdriver 212 for a time period T1. After T1 has passed, microprocessor 213“flips” the first and second power array for a time period T2. In otherwords, as seen in FIG. 8, after T1 ends and T2 begins, the first powerarray is delivered to the second triac driver 212 and the second powerarray is delivered to the first triac driver 211.

Microprocessor 213 performs the step of calculating T1 and T2 as:

T1=2 seconds*(First heater total power/Combined heater total power)

T2=2 seconds*(Second heater total power/Combined heater total power).

Mathematically, it follows that the power delivery phase of T1+T2=2seconds.

In this way, the power arrays are delivered for a combined powerdelivery phase of 2 seconds. It is contemplated that longer or shorterpower delivery phases may be used. After 2 seconds, microprocessor 213may re-calculate the power arrays. By re-calculating the power arrays,microprocessor 213 may account for a change in user settings, or toswitch from raising a heating element's temperature to maintaining atemperature.

An operating example applying the devices and methods described above isprovided. For example, a user may wish to use the grill 110 withdifferent power levels for the first and second heating elements 203 and204—for instance, microprocessor 213 may determine that a first heatingelement 203 should have 17.5% of its maximum power, and a second heatingelement 204 should have only 5% of its maximum power. In accordance withthe embodiments described herein, microprocessor 213 is configured todeliver 17.5% and 5% power, respectively, while drawing power in apattern that reduces the harmonic currents introduced by the electricgrill into the AC wall outlet.

In this example, the first and second power arrays are calculated asfollows: the first and second selected power levels are combined toarrive at a total selected power: 17.5%+5%=22.5%, or 0.225 (See 603).Because this is less than 50%, microprocessor 213 proceeds with step605. Using the techniques described herein, microprocessor 213multiplies by eight (8) to arrive at 0.225*8=1.8. Next, microprocessor213 fills the value 1.8 into the first power array. In particular, thefirst cell and third cells receive the value of (1.8)/2=0.9. The secondand fourth cells remain “0.” Thus, the first power array is“0.9|0|0.9|0” and the second power array is “0|0|0|0.”

For a time period T1, the first power array is delivered to the firsttriac driver 211 and the second power array is simultaneously deliveredto the second triac driver 212. In delivering the first and second powerarray, microprocessor 213 sends an “on” signal to the respective triacdriver 211 and/or 212 at a time that corresponds to the “cut” of thewave. For example, the first power array's first cell dictates that a90% power wave (i.e. 0.9) is delivered. A 90% power wave requires a“cut” angle of arccos (2*9−1)=36.86°. Microprocessor 213 delivers a 90%power wave by turning triac driver 211 “on” at 36.86°. Similar to thevalues of Table 1, a 36.86° cut can be made by delivering power 0.0017seconds after a zero crossing. Subsequently, the second cell dictatesthat an “off” wave having 0% is delivered. The third wave is the same asthe first wave, i.e. cut at 36.86°, and the fourth wave is the same asthe second wave, i.e. “off.” The second power array in this example is“0|0|0|0,” thus the second triac driver 212 is never activated.

This delivery pattern is continued for a time period T1 as described at612 and 613. Here, T1 is calculated as T1=2 seconds*(First heater totalpower/Combined heater total power)=2*(0.175/0.225)=2*0.78=1.56 seconds.Similarly, T2=2*(0.05/0.225)=0.44 seconds. In this example, the firstpower array (“0.9|0|0.9|0”) is delivered to the first heating element203 and the second power array (“0|0|0|0”) is delivered to the secondheating element 204 for T1=1.56 seconds. After 1.56 seconds,microprocessor “flips” the delivery of the first and second power arrayfor a period of 0.44 seconds. After a combined 2 seconds have passed,microprocessor 213 may begin by re-filling the first and second powerarray according to the power needs at that point in time.

It will be understood that microprocessor 213 may include internal orexternal memory 1000 for reading and/or writing in connection withexecuting the steps and configurations described herein. Moreover, itwill be understood that microprocessor 213 may have an internal orexternal clock signal that may be used to time the “on” signal sent to atriac. The clock signal may be generated by an on-board clock signalgenerator 1001, or by an external clock. FIG. 10 is an exemplaryschematic showing inputs and outputs to microprocessor 213. Examplesinclude a left and right knob 101, 102 and a display 103. Additionalexamples include thermocouples 221, 222, and communication with triacDrivers 208 and 209. Memory 1000 and clock 1001 are also shown, as isthe input signal 1002 from zero crossing unit 210.

An additional benefit of embodiments of the devices and methodsdescribed herein is a reduction in flicker introduced by the digitalpower supply 200 into a wall outlet. Flicker is undesirable because, atcertain frequencies, it will cause lights connected to an outlet toflicker or dim. FIG. 11 shows the flicker limits of IEC61000-3-3Electromagnetic compatibility (EMC)-Part 3-3 (VoltageFluctuations and Flicker). Flicker is measured as a % change in voltage.

Embodiments of the present invention may reduce flicker levels to a walloutlet based on voltage changes resulting from wave-cuts within a singlepower delivery phase. A person of skill in the art would recognize thatflicker is commonly measured during a devices “steady state.”

The voltage changes within a single power deliver phase comply with theflicker regulations. As seen at 1101 (and further described in thestandard), the IEC 61000-3-3 requirement's last data point occurs at2875 voltage changes per minute. This equates to a cycling frequency of23.96 Hz. In other words, voltage changes occurring at a frequency above23.96 Hz have no flicker requirement because they are beyond humanperception. Embodiments of the devices and methods disclosed hereincreate a wave pattern in which electric grill 110 alternates between acut wave and a full “on” or a full “off” wave. Following this pattern,electric grill 110 would create 25 voltage changes per second (25Hz) at50 Hz AC and 30 voltage changes per second (30 Hz) at 60 Hz AC. A cutwave followed by a full wave counts as one voltage change. The 25 Hz and30 Hz cycling frequencies are above the standard's last data point of23.96 Hz and therefore comply with flicker requirement.

An additional benefit of embodiments of the invention comes fromsplitting power into multiple power arrays and delivering them tomultiple heating elements. Using the techniques described in FIGS. 6 and7, one of the power arrays will always be “0|0|0|0” or “1|1|1|1.” Thisensures that only one of heating element 203 or 204 can receive a “cut”wave at any given time. As a result, the electric grill 110's usedcurrent (or power) will never be dropped by more than half (½) of themaximum rated power. To give an example, if two heating elements 203 and204 each draw 1150 Watts, for a combined 2300 Watts drawn by theelectric grill 110, then even a 90° in one heating element 203 or 204would only result in a maximum power drop of 1150 Watts. This helpsreduce the magnitude of harmonic currents.

Embodiments of the disclosed digital power supply and method fordelivering power may optionally be implemented in the circuitry of anelectric grill. FIG. 2 shows additional components that may optionallybe added to the protection circuitry 200 to provide circuitry for anelectric grill. For example, line 201 and neutral 202 may connect to astep down transformer 215 to which zero crossing detection unit 210 isconnected. Step down transformer 215 provides a reduced secondaryvoltage so that zero crossing detection unit 210 may detect zerocrossings in AC current between line 201 and neutral 202 without beingexposed to high voltages.

Further optional embodiments include a full wave rectifier 216 thatfeeds to a ground fault detection unit 217, which in turn communicateswith a trip controller 218 for tripping an electromechanical latch 206or 207. Ground fault detection unit 217 may receive a signal indicatinga current imbalance between line 201 and neutral 202 and cause thelatches to trip to prevent hazardous current situations.

Additional optional embodiments include a watchdog monitor 220 whichmonitors the operation of microprocessor 213 and may disable triacdrivers 211 and 212 in the event of a failure of microprocessor 213.Also provided are AC/DC power converters 214 which may be used to powerthe microprocessor 213, and a current sensor, such as Hall Effect sensor219, which may be used by microprocessor 213 to monitor the currentflowing to heating elements 203 and 204.

For the reasons described above, some embodiments of the inventions mayprovide a digital power supply that increases a heating element'slifespan; complies with flicker requirements, and also complies withharmonic requirements. These benefits may be accomplished using thedevices and methods described herein. For example, using a powerdelivery phase of 2 seconds prevents the heating elements from everfully expanding or fully contracting. Lengthy power delivery phases thatallow a heating element to fully expand or contract are very detrimentalto the heating element's lifespan. The flicker requirement is satisfiedby creating a total power array that describes an alternating wavepattern which has a cycling frequency of 25-30 Hz depending on the ACcurrent. Moreover, the total power array that may be created usingdevices and methods of the invention follow every cut wave with a full“on” or full “off” wave, thus reducing harmonic currents. Harmoniccurrents are also reduced by splitting the combined load of electricgrill 110 to two or more elements.

The above description is not intended to limit the meaning of the wordsused in or the scope of the following claims that define the invention.Rather the descriptions and illustrations have been provided to aid inunderstanding the various embodiments. It is contemplated that futuremodifications in structure, function or result will exist that are notsubstantial changes and that all such insubstantial changes in what isclaims are intended to covered by the claims. Thus, while preferredembodiments of the present inventions have been illustrated anddescribed, one of skill in the art will understand that numerous changesand modifications can be made without departing from the claimedinvention. In addition, although the term “claimed invention” or“present invention” is sometimes used herein in the singular, it will beunderstood that there are a plurality of inventions as described andclaimed.

Various features of the present inventions are set forth in thefollowing claims.

What is claimed is:
 1. A method of delivering power, comprising the steps of: using one or more user input devices to select a first and second power setting for a first and second heating element, respectively; electronically communicating the power settings to a microprocessor and using the microprocessor to calculate a total amount of power requested; using the microprocessor to populate a first and second power array corresponding to the first and second heating element, respectively; using the microprocessor to calculate a first and second phase angle array corresponding to the first and second power array; causing the microprocessor to receive a zero crossing signal from a zero crossing detection unit; and for a first time period, delivering a phase-controlled AC wave pattern represented by the first phase angle array to the first heating element and delivering a phase-controlled AC wave pattern represented by the second phase angle array to the second heating element.
 2. The method of claim 1, further comprising the step of: for a second time period, delivering a phase-controlled AC wave pattern represented by the first phase angle array to the second heating element and delivering a phase controlled AC wave pattern represented by the second phase angle array to the first heating element.
 3. The method of claim 2; wherein each power array contains four cells.
 4. The method of claim 3; wherein the step of using the microprocessor to populate a first and second power array further comprises the steps of: populating the first cell of the first power array with the same value as the third cell of the first power array; populating the second cell of the first power array with the same value as the fourth cell of the first power array; populating the first cell of the second power array with the same value as the third cell of the second power array; and populating the second cell of the second power array with the same value as the fourth cell of the second power array.
 5. The method of claim 4, wherein each cell of each power array represents a power percentage and ranges from 0≦x≦1.0.
 6. The method of claim 5, wherein every alternate cell in the first power array is populated with a “0” or a “1”.
 7. The method of claim 6, wherein every alternate cell in the second power array is populated with a “0” or a “1”.
 8. The method of claim 5, wherein the first and second phase angle arrays are calculated using the microprocessor to apply the equation angle=arccos(2x−1) to the first and second power array, respectively.
 9. The method of claim 5, where in the first time period is calculated as a ratio of the first power setting to the total amount of power requested and the second time period is calculated as a ratio of the second power setting to the total amount of power requested.
 10. The method of claim 5, wherein the step of delivering power further comprises the step of activating a triac connected to a heating element.
 11. A digital power supply, comprising: a first and second user input; a first and second triac connected to a voltage line; a first and second triac driver respectively in communication with the first and second triac; a microprocessor in communication with the first and second triac drivers and in communication with the first and second user input; wherein the microprocessor is specifically configured to calculate a total power requested by the first and second user inputs and to populate a first and second power array based on the total power requested; and wherein the microprocessor is specifically configured to calculate a first and second array of phase angles based on the respective values of the first and second power array.
 12. The digital power supply of claim 11, wherein the first and second power array each have four cells.
 13. The digital power supply of claim 12, further comprising; wherein the microprocessor is specifically configured to populate at least one power array's cells with two alternating values.
 14. The digital power supply of claim 11, further comprising the microprocessor being configured to turn on the first and second triacs in a timing pattern that corresponds to a phase-controlled wave form in the first and second phase angle arrays.
 15. An electric grill, comprising: a first knob, a second knob, and a display mounted on a housing; a power cable connected to a voltage line and a neutral line; a first and second heating element inside the housing, the first and second heating elements being connected to the voltage line and the neutral line; a first and second triac connected between the voltage line and the first and second heating elements respectively; a first and second triac driver respectively in communication with the first and second heating elements; a zero crossing detection unit configured to detect zero crossings of AC current in the voltage line; and a microprocessor in communication with the first and second knob, the first and second triac drivers, and the zero crossing detection unit, wherein the microprocessor further communicates with a clock signal generator and a memory.
 16. The electric grill of claim 15, wherein the memory contains a first and second power array.
 17. The electric grill of claim 16, wherein the first power array is populated with two alternating values.
 18. The electric grill of claim 17, wherein the second power array is populated with two alternating values.
 19. The electric grill of claim 18, wherein one of the two alternating values in the first power array represents a full “on” wave.
 20. The electric grill of claim 18, wherein one of the two alternating values in the first power array represents a full “off” wave. 